Fix BOLA rule on track switch (#3884)

* BOLA rule: reset placeholderbuffer on empty buffer only for concerned media type

* BOLA rule: if audio buffer runs empty (due to track switch for example) then reset placeholder buffer only for audio (to avoid decrease video BOLA quality)

src/streaming/rules/abr/BolaRule.js

@@ -39,6 +39,7 @@ import EventBus from '../../../core/EventBus';
 import Events from '../../../core/events/Events';
 import Debug from '../../../core/Debug';
 import MediaPlayerEvents from '../../MediaPlayerEvents';
+import Constants from '../../constants/Constants';
 
 // BOLA_STATE_ONE_BITRATE   : If there is only one bitrate (or initialization failed), always return NO_CHANGE.
 // BOLA_STATE_STARTUP       : Set placeholder buffer such that we download fragments at most recently measured throughput.
@@ -258,9 +259,12 @@ function BolaRule(config) {
         checkBolaStateStableBufferTime(bolaState, mediaType);
     }
 
-    function onBufferEmpty() {
+    function onBufferEmpty(e) {
         // if we rebuffer, we don't want the placeholder buffer to artificially raise BOLA quality
-        for (const mediaType in bolaStateDict) {
+        const mediaType = e.mediaType;
+        // if audio buffer runs empty (due to track switch for example) then reset placeholder buffer only for audio (to avoid decrease video BOLA quality)
+        const stateDict = mediaType === Constants.AUDIO ? [Constants.AUDIO] : bolaStateDict;
+        for (const mediaType in stateDict) {
             if (bolaStateDict.hasOwnProperty(mediaType) && bolaStateDict[mediaType].state === BOLA_STATE_STEADY) {
                 bolaStateDict[mediaType].placeholderBuffer = 0;
             }